This invention relates generally to switching mechanisms and more particularly to a proximity switch for use in United States Postal Service Multi-Position Letter Sorting Machines (MPLSMs).
It has been recognized that mechanical switches employing moving parts exhibit limited life cycles. While such a limited life cycle may be adequate for many applications due to infrequent use of the switch, relative ease of replacement or low cost, certain applications require a switch having an extended life cycle. One such application is in the area of letter sorting machines such as the MPLSM manufactured by Burroughs for use in sorting letter mail by the U.S. Postal Service. Such letter sorting machines generally have a plurality of consoles each operated by a keypunch operator. Consoles are provided with keyboards which typically comprise an upper row of ten keys and a lower row of ten keys. Each row of ten keys corresponds to digits zero through nine. The lower row of keys is generally utilized for ZIP code entry while the upper row of keys is generally utilized for prefix entry. Prefix entry typically comprises depressing a single key of the upper row of keys which corresponds to the general geographical destination of the specific letter. For example, "1" may be depressed on the upper row of keys for any destination within New York City, which generally is associated with numerous ZIP codes.
As pieces of mail are automatically brought by a mechanical arm of the MPLSM into view of the operator, the operator visually identifies a portion of the ZIP code. In a specific case of final mail sorting by MPLSMs manufactured by Burroughs the operator generally reads the last two or three digits of the ZIP code and inputs their value into the lower keyboard by depressing the corresponding keys in sequential order. For example, if the ZIP code were 11385, the operator would read "85" and depress the ninth key on the lower row of ten keys and then depress the sixth key also on the lower row of ten keys. Alternatively, the operator may enter the "8" on the lower row of keys and the "5" on the upper row of keys. In this specific case, mail arriving at the console has been presorted according to the first three digits in a five digit ZIP code.
Unfortunately, all known letter sorting machines presently in use by the U.S. Postal Service are provided with a mechanical switch of the miniature snap limit type associated with each of the twenty keys of a keyboard. The life cycle of such switches is generally on the order of one million cycles, corresponding to a mean time of failure just two months after installation. Considering that a four console machine typically contains 80 such switches, and that failure in a single switch necessitates shutting down the entire console, it is clear that a more reliable switch is desirable.
The inadequacy of mechanical switches presently used in letter sorting machines is further exemplified by the existence of approximately four regional rebuilding centers devoted largely to refitting keyboards having defective mechanical switches with new mechanical switches.
An additional deficiency of existing mechanical switches relates to the electrical phenomenon of "bounce". Upon urging a first contact in a mechanical switch into electrical contact with a second contact, a clean, single transition from one voltage level to another voltage level generally does not occur. Rather, numerous transitions will occur among various voltage levels prior to stabilization at the second level. To remedy this deficiency, existing letter sorting machines are provided with additional software programs and/or hardware to prevent the false results due to bounce. Advantageously, a system wherein such bounce does not occur would not require additional compensating hardware and/or software which increases costs and reduces reliability.